The most usual bandpass filters made from rectangular section waveguide are constituted by a series of resonant cavities each cavity having a length about .lambda. go/2, where .lambda. go is the length of the guided wave at the center frequency Fo of the filter. The first and last cavities are each connected to a respective length of standard inlet or outlet waveguide. Coupling between the successive resonant cavities, and between the end cavities and the lengths of inlet or outlet waveguide is performed by coupling irises. In known arrangements, such filters have a linear structure obtained by inserting obstacles into a rectangular section waveguide to partition the wave guide longitudinally into portions whose lengths are about .lambda. go/2, or a multiple thereof. These portions constitute the resonant cavities which are interconnected in series and with the inlet and outlet lengths of waveguide via coupling apertures or irises in the obstacles. The dimensions of the rectangular cross-section of the waveguide are standardised as a function of the type of filter to be obtained and in terms of the desired center frequency Fo and bandwidth.
Such filters are reversible, that is to say either one of the end lengths of waveguide may be used as the inlet, with the other being used as the outlet. Such filters may be designed using the methods explained in "Microwave filters, Impedance matching networks, and Coupling structures" by G. L. Matthaei, L. Young, and E. M. T. Jones, published by the McGraw-Hill Book Company, New York, with particular reference to pages 434, 450, 461, 463, and 234.
Bandpass filters are particularly used in filtering assemblies where the center frequencies of the various filters are shifted with respect to each other according to the frequency channels they are to combine, i.e. assemble or separate.
In this type of application, it is known to use bandpass filters of adjustable center frequency. One known solution for providing filters of adjustable center frequency is given by U.S. Pat. No. 3,130,380 in the name of David F. Bowman. This specification describes cavities which are all of the same width .lambda. g/2, said width being delimited by the distance between a pair of pistons. The resonant frequency of the cavities is set by varying the distance between each pair of pistons, while coupling variations are obtained by shifting the pistons in pairs to provide different off-sets for the inlet and outlet coupling irises with respect to the different cavities. The dimensions of the coupling irises can also be varied to help improve the filter characteristics.
Preferred elbodiments of the present invention provide a bandpass filter with a defined filter characteristic that can be shifted over a wide range of frequencies with very little variation in the said frequency characteristic. This can be obtained without modifying the mechanical structure of the filter and without modifying the positions and/or sizes of the coupling irises.